Apparatus for motor current minimization

ABSTRACT

Current minimization in a motor control system is obtained through a voltage control where the amplitude of the applied voltage is increased or decreased each time measured motor current passes through a detected minimum current level.

United States Patent 1191 Opal et al. 1 1 Mar. 27, 1973 541 APPARATUSFOR MOTOR CURRENT 3,611,089 10 1971 Mokrytzki et al. ..3l8/230 xMINIMIZATION 3,577,176 5 1971 Kreithcn ct al ..3l8/432 PrimaryExaminer-Bernard A. Gilheany Assistant Examiner-W. E. Duncanson, Jr.Attorney-Brown, Murray, Flick & Peckham [73] Assignee: Power ControlCorporation, Pittsburgh, Pa. [57] ABSTRACT [22] Filed; Jan, 21, 1972Current minimization in a motor control system is obtained through avoltage control where the amplitude [21] Appl 219703 of the appliedvoltage is increased or decreased each time measured motor currentpasses through a de- 52 U.S. Cl ..31s/432 tected minimum current level.[51] Int. Cl. ..H02p 5/28 [58] Field of Search ..3l8/230, 432, 433, 434

[5 6] References Cited 8 Claims, 7 Drawing Figures UNITED STATES PATENTS3,624,837 l1/l97l Albarda ..3l8/434 X 200 I00 Foul 3 2 VOLTAGE AND MOTORGENERATOR MLL L i 1E6 l/ou! lisx-z r T 300 CURRENT M/N IMUM IZA 7/0/1/CIRCUIT Patented March 27, 1973 3 Sheets-Sheet l l lxmwk m. WEN N SEN ms9 $6 :QRY \D "KG N fi 535 2st 5 5:53; g kammmau Q% h m xw u I T|..||||J2 u u v Wm 53 QQRYQWEMQ muammmmmm 33 Ill: .55 n xuzmamwmm @EQ Q5 mmE w m3: m9 saw Patented March 27, 1973 I mm V Vs Holding Ie CL Follows I toImin 3 Sheets-Sheet g CU Holding Ie CL Holding Imin PL Triggers 0 when IImin+Ime ZA-l I 0P=i0 I Imin Imin 0R=// Ime Ime V V CL Holding Im CLHolding Im CU Follows I '70 I'm/n CL Holding Imin PU Triggers 0 when IIm1n Ime PU Triggers 0 PL Triggers 0 I I I Ime Im [me Im Ime Ime 0R= 000R=0l 0R=l0 0R=ll L y l Z J K J l 28-0 ZB-l 28-2 28-3 MODE CODE MOI? 0.STEAD) STA TE CUHRENTMI/V/MUM/ZAT/O/V 4 I I CU Holds Minimum I (Im) 260 M00 f Decreases CL Follows I to Minimum l 0ecreases I PL Tri ersO when20/ M 99 0/ on PL pu/se increases I CL (Inf) I CU F v I I M o ows 0In/mum 2C2 MIG I Decreases 7 CL Holds Minimum Increases M H00 I PUTriggers 0 when Increases I CU an PUPu/se ra'Moo Pat ented March 27,1973 3 Sheets-Sh e APPARATUS FOR MOTOR CURRENT MINIMIZATIONCROSS-REFERENCES TO RELATED APPLICATIONS (I) Application Ser. No.219,704, filed Jan. 21 1972 and entitled Method and Apparatus forProviding Efficient and Stable Power Inversion with Voltage andFrequency Control.

BACKGROUND OF THE INVENTION In motor control systems of the typedescribed in copending application constituting Reference (1) above,provision is made to determine the minimum current required to maintainconstant torque for a given load condition and, as the motor or loadchanges, the system continuously seeks the new current minimum andregulates about this level. It has been found that as the voltage isvaried under constant load conditions, motor current decreases as thevoltage is increased from a relatively low level and reaches a minimumvalue with further increases in voltage thereafter causing the currentto increase. In low load operating conditions, it has been found thatmotor current may be decreased as much as 40 percent while stillmaintaining constant torque. Even under heavy load conditions, thecurrent reduction may still be as high as percent.

SUMMARY OF THE INVENTION The invention provides a method and apparatusfor automatically finding the optimum motor operating point wherecurrent is held to within 200 milliamperes of its minimum value whilestill maintaining constant torque. The result is a considerablereduction in motor losses and improved efficiency in the control systemwhich may be of the type described in the aboveidentified copendingapplication.

According to the basic method of the invention, first and seconddirectional sample and hold circuits are utilized to detect minimumcurrent conditions during periods of increasing and decreasing appliedvoltage. The output signals provided by the sample and hold circuits arecompared in respective differential amplifiers with a signalrepresenting motor current. Whenever the current crosses the level ofthe sampled current minimum in an increasing direction, a trigger pulseis generated which causes the voltage applied to the motor to changeaccordingly. Thus, if during increasing voltage, current decreases to aminimum with a sample being held in the first sample and hold circuit,then after the current rises above this minimum, the voltage isdecreased, with the sampling function being transferred to the secondsample and hold circuit.

Since the invention may be employed in a system where the initialdirection of voltage movement (increasing or decreasing) is not known,provision is made, according to the invention, to establish an uppercurrent limit referred to herein as Ie. The method of control thenprovides that whenever the current is caused to exceed an initialcurrent condition (Is), which is less than Ie, the direction of voltagemovement is reversed. Thus, during the initial operation of theinvention, the current level Is is utilized to trigger the voltage statewhereas, once the minimum current has representing cation, and in which:

FIG. 1 shows a system employing the invention;

FIG. 2 shows a curve relating voltage to current for a typical motor;

FIG. 2A shows a set of curves illustrating the method of operation ofthe invention under initial error conditions;

FIG. 2B shows a set of curves illustrating a method of operatingaccording to the invention to maintain a stable minimum currentcondition;

FIG. 2C is a chart summarizing the modes of control during steady-statecurrent minimization;

FIG. 3 is a block diagram of apparatus for carrying out the currentminimization method of the invention; and

FIG. 4 is a schematic diagram showing specific circuits which aresuitable for providing the function of the means shown in FIG. 3.

In FIG. 1, a load 1 driven by a motor 2 is controlled through a motorcontrol in response to output voltage (Vout) and frequency (Four)reference signals which are initially provided by a voltage andfrequency reference generator 200. A detailed description of thespecific structure suitable for means 100 and 200 is found in Reference(1 above. The current of motor 2 is sensed through a suitable transducer3 producing representation VI having a level corresponding to the sensedcurrent. As is more fully explained in Reference (1), the motor isstarted up under control of means 200 in response to a frequency setsignal (Fsel). At this time, a switch SX with contacts SXl and 8X2, isin its normally closed state so that the voltage reference signal Vrefis applied through contact SXI to a capacitor CVout producing referencesignal Vout applied to motor control 100. When Fout reaches Fset, switchSX is actuated to transfer voltage control to the current minimizationcircuit 300 of the invention. At this time, SX2 also closes providing anegative pulse for an error signal generator within means 300 toestablish an upper current level reference. This upper level is referredto herein as Ie.

Before considering the specific apparatus utilized to practice themethod of the invention, the basic method will be described withreference to FIG. 2. It will be noted that four possible operatingconditions are set forth as related to the voltage-current curve for themotor. Since the control logic utilized in carrying out the method ofthe invention is based upon the logical definition of the operatingconditions, they have been translated into an operating code referred toas OR. The representation Q is ON, or in a I state, when voltage isincreasing and is OFF or in the 0 state when voltage is decreasing. Therepresentation R corresponds to the increasing and decreasing directionsof current so that when current is increasing, R I, and when current isdecreasing, R 0. Thus, it will be noted that two operating conditionsexist to the left of the minimum current level Im referenced as OR 01,when current increases for decreasing voltage, and operating conditionQR 10, when current decreases for increasing voltage. To the right ofthe minimum current level Im, the two operating conditions are: QR 11when both current and voltage increase; and QR when both voltage andcurrent decrease.

Reference is now made to FIG. 2A to illustrate the basic method ofcontrol starting from an initial condition where the initial currentlevel is represented as Is and the initial voltage level is representedas Vs. This initial state is assumed to be to the right of the currentminimization point. The system is assumed to start operation in mode QR00 and voltage is decreased. During this time, a first directionalsample and hold circuit (referred to sometimes as upper circuit CU)holds a value corresponding to Ie, and a second sampling circuit(referred to sometimes as CL) follows the sensed current, represented bysignal VI, to the minimum level Im. As the voltage then decreasesfurther, current increases during mode QR 01 (FIG. 2A-l) until thedetected current exceeds the minimum level (Im) sampled by circuit CL.When this happens, a circuit referred to as PL, triggers reference Q inorder to reverse the direction of voltage variation. Thus, mode QRfollows (FIG. 2A-2) with increasing voltage and decreasingcurrent,'during which time circuit CU follows the measured current downto level Im while CL holds the previously detected minimum value Im. Thefinal mode occurs when the minimum point is passed and both voltage andcurrent increase. This is terminated when the current exceeds theminimum level Im by a maximum error referenced as Ime. This triggers anupper pulse circuit referenced as PU, which causes Qto reverse again andthe system then re-enters mode OR 00.

The operation of the system, following the establishment of the minimumcurrent level Im, may be considered to be a steady state series of modesand is sum marized in the curves of FIG. 28. During mode QR 00 (M00),both voltage and current decrease with the function of the upper andlower circuits being summarized in FIG. 2C-0. Specifically, the uppercircuit holds the last minimum current sample Im which was detectedduring the increasing voltage period (Q l) which preceded mode M00. Itmay be considered that this was the sample and hold function of circuitCU during modes M10 and M11 shown in FIGS. 2A-2 and After mode M00, modeM01 is entered, characterized by a reversal in the trend of the currentwhich begins to increase. This mode is terminated when the measuredcurrent exceeds Im and circuit PL istriggered reversing both Q and R tocause entry into mode M10. During this mode, voltage increases andcurrent decreases with circuit CU following current to the minimum andcircuit CL holding the previous minimum. Mode M11 is then entered whencurrent starts to increase again being terminated when the current levelexceeds the minimum held by circuit CU causing a pulse circuit PU totrigger Q and to re-enter mode M00 previously discussed.

Thus far, only one of the four possible starting states have beenconsidered where current was at an initial level Is for a voltage Vs andthe initial mode was assumed to be QR 00. The following provides asummary of the other three possible starting states:

' SUMMARY OF OTHER STARTING STATES OR 01 PL triggers 0 when I Is I OR 10CU follows to lm; CL holds Is OR II PU triggers when I Im QR 00 CU holdsIm; CL follows to Im OR OI PL triggers Q when I lm OR 10 CU follows toIm; CL holds Ie QR =11 PU triggers Q when I Im OR 00 CU holds Im', CLfollows to Im OR 01 PL triggers Q when I Im OR II PU triggers 0 when IIs OR 00 CU holds Is; CL follows to Im OR 01 PL triggers Q When l Im OR10 CU follows to lm; CL holds Im OR II PU triggers Q when I Im It willbe noted that in all cases of increasing voltage (Q 1), the uppercircuit CU follows the current to the minimum value and the lowercircuit CL holds the previous sample which may not be at the minimumduring the initial starting condition. In the case where voltage isdecreasing (Q 0), it is circuit CL that follows the current to theminimum Im and circuit CU during that time holds the previous sample. Inall cases, after the initial modes, the system becomes stabilized aboutthe steady-state current minimization modes M00-Mll summarized in FIG.2C.

The general organization of apparatus for carrying out the method of theinvention is set forth in FIG. 3

where a sample error pulse generator 310 is initially actuated by switchSX2 to establish the maximum current error level Ie. A specific circuitfor accomplishing this will be considered with reference to FIG. 4. Twodirectional sample and hold circuits 320U and 320L receive arepresentation of the current VI through respective gated samplingamplifiers 330U and 330L controlled by signals Q and Q, respectively.Thus,

when voltage is increasing, amplifier 330U is turned ON and sample andhold circuit 320U then follows the current representation whileamplifier 330L is biased OFF by signal 0' so that sample and holdcircuit 320L functions to hold its previous sample. These operatingconditions are reversed when the state of signal Q changes from a 1 to0, in which case the lower sample and hold circuit 320L becomesoperative to follow the current representation and the upper sample andhold circuit .then holds its previously sampled minimum value. Theoutput signals of circuits 320 are applied to respective differentialamplifiers 340U and 340L which also receive the current representation.Amplifiers 340 have their outputs applied to respective gated pulseamplifiers 350U and 350L controlled by signals 0 and Q, respectively.When voltage is increasing, it is the upper pulse amplifier 350U that isrendered operative so that as soon as the sensed current level exceedslevel Im held in circuit CU, an output pulse PU is generated which thentriggers the state of a flip-flopQ (360) switching to the next mode ofoperation. In a similar manner, when voltage is decreasing, amplifier350L becomes operative to generate a pulse PL when the sensed currentlevel exceeds Im held by the lower circuit.

The output signals of circuit O, which may be a flipflop, are utilizedto turn ON either an upper current source 370U (also referenced as CSU)or a lower current source 370L (also referenced as CSL). Thus, whenvoltage is to be increased and signal 0 is ON, source CSU charges upcapacitor CVout, and when flip-flop Q is OFF, lower current source CSLis effective to discharge capacitor-Vout.

Means 310 of FIG. 3 is provided by an equivalent circuit 310 in FIG. 4where switch signal SX2 causes a pulse to be generated through acapacitor CSL, forward-biasing the base of a transistor TlL to charge aholding capacitor C2L in lower sampling circuit 320L. A similar initialcharge, representing the current error 12, is passed through a capacitorCSU in circuit 310 to the base of a transistor TlU causing it to beforwardbiased and to transmit a charging pulse representing Ie to anupper holding capacitor C2U in circuit 320U. Thus, at the start ofoperation, the representation of signal Ie is applied to both upper andlower circuits. During the following circuit operation, however, theerror representation is sent only to that circuit which is to operate tofollow the current representation VI. Thus, when signal Q changes froman ON state to an OFF state, a negative pulse passes through capacitorClL in circuit 310 to bias ON transistor TIL and, at the same time,gated sampling amplifier 330L is turned ON by Q as signal Q goes OFF.Within circuit 330L, a gate input G3L receives signal Q and is operativeto forward-bias a transistor T3L so that the current representation VImay be sampled through a diode D3L. In a similar manner, when signal Qfirst turns ON, its complementary signal Q turns OFF, sending a negativepulse through capacitor ClU in circuit 310 to forwardbias transistor TlLto send a pulse representing Ie to capacitor C2U and gated samplingamplifier 330U is turned ON by signal Q through a gate input G3U appliedto the base of a transistor T3U, the emitter of which receives thecurrent representation Vl through a diode D3U.

During periods of increasing voltage (Q l), the upper circuit thenfunctions to follow or sample the current representation and todischarge capacitor C2U to the minimum current value. Capacitor C2Ucannot go below the minimum value because of the biased direction ofamplifier 330U and thus, as current passes the minimum level and rises,capacitor C2U remains at the lowest sampling level. During the timevoltage is increasing in this manner, circuit 330L is OFF and capacitorC2L holds whatever level was previously established during its samplingperiod. In a similar manner, when voltage is decreasing, the lowercircuit is operative to follow the current representation whereas theupper circuit holds the previously detected minimum current level.

Upper and lower differential amplifiers 340, referred to as DAU and DAL,operate in the same manner to detect the crossing of the currentrepresentation VI over the sampled current level. The sampled level madeduring initial operation may be higher than the minimum current, but itis eventually brought to the minimum level after several modes, aspreviously described. The differential amplifiers are operative toproduce pulses for crossings in either direction with the controlselecting the proper trigger pulse being obtained in gated pulseamplifiers 350. Amplifier 350U receives signal Q so that the upper pulseamplifier becomes operative only when voltage is increasing (Q 1) andspecifically for the state QR ll or mode M11.

A suitable arrangement for pulse amplifier 350 is shown where a diode D5receives the gating signal at its cathode and the anode thereof isconnected to a resistor R5 which receives the differential amplifieroutput signal. The junction of the resistor and the diode is utilized toturn ON a pulse amplifier to produce the desired trigger pulse. When thegating signal, such as Q, is at a low level, the associated diode, inthis'case DSU, holds the amplifier input to a reference level such asground, preventing any trigger pulse from being generated.

From the foregoing description, it should now be apparent that theinvention provides an effective method for automatically detecting andstabilizing a control signal at that level which will assuresubstantially minimum motor current. While specific circuits have beenshown, it will be understood that the basic concept may be practicedwith a large variety of circuits.

The concept of current minimization may be generalized to cover anysituation where a control signal may be varied to minimize or maximizethe level of an output or load signal. The stabilization point in allcases occurs where the derivative of the signal if substantially zero.Although, theoretically, the method of the invention makes it possibleto hold the operating point at precisely zero, in actual practice, asmall error between lm and [me will exist due to the hysteresischaracteristic of the differential amplifying means which are employed.It has been found in actual practice, however, that the error in thestabilization is a very small percentage of the total reduction of theoutput signal, (specifically current).

I claim as my invention:

1. In a motor control system wherein motorcurrent is caused to decreasefor increasing applied voltage until a minimum current point is reachedand then increases with increasing applied voltage thereafter, thecombination comprising: first and second directional sample and holdmeans for detecting minimum current conditions during periods ofincreasing and decreasing input voltages; first and second gated pulsemeans for producing trigger pulses when the current rises above saidminimum current point during periods of increasing and decreasingvoltages; and voltage level varyingmeans responsive to said triggerpulses for changing the direction of voltage variation each time atrigger pulse is received.

2. In the system of claim 1 wherein there is included means forestablishing a predetermined initial voltage in a capacitor and meansfor producing a switching signal to initiate current minimization, saidcombination including sample error pulse generating means for entering apredetermined upper current limit signal into said sample and holdmeans.

3. The combination of claim 2 wherein trigger pulses are produced toreverse voltage direction when the motor current reaches an initialupper current value;

4. The method of varying voltage in a motor control system to minimizemotor current comprising: decreasing voltage until motor current crosseseither a minimum, or maximum error level while current is increasing;increasing voltage until motor current crosses either a minimum, ormaximum error level with current again increasing; and maintainingsteady-state current minimization by reversing the direction of voltagevariation each time motor current decreases to a minimum and then risesslightly above said minimum by a maximum error value.

5. A method for maintaining steady-state current minimization in a motorcontrol system comprising: establishing four control modes for theconditions of: voltage and current decreasing; (1) voltage decreasingwith increasing current; (2) voltage increasing with decreasing current;and (3) voltage and current both increasing; operating a first circuitto hold a signal representing minimum current and a second circuit tofollow current to the minimum during mode (0); comparing a signalrepresenting motor current with the signal held by said second circuitduring mode (1) and switching to mode (2) when motor current exceeds theminimum current held by said second circuit; operating I tain level, amethod for automatically varying said control signal to cause the outputsignal to assume a minimum level where the derivative of said outputsignal is substantially zero, said method comprising sampling the levelof said output signal as its derivative decreases during periods ofincreasing control signal and holding the zero derivative level of saidoutput signal during periods of decreasing control signal to produce afirst sample and hold sample; sampling the level of said output signalas its derivative decreases during periods of decreasing control signaland holding the zero derivative level of said output signal duringperiod of increasing output signal to produce a second sample and holdsignal; and changing the direction of control signal variation each timethe output signal to level crosses the level of either of said sampleand hold signals.

7. The method of claim 6 wherein said output signal is motor current andsaid control signal is voltage.

8. The method of claim 7 wherein first and second trigger pulses aregenerated when the current level sampled crosses the level of said firstand second sample and hold signals, respectively.

1. In a motor control system wherein motor current is caused to decreasefor increasing applied voltage until a minimum current point is reachedand then iNcreases with increasing applied voltage thereafter, thecombination comprising: first and second directional sample and holdmeans for detecting minimum current conditions during periods ofincreasing and decreasing input voltages; first and second gated pulsemeans for producing trigger pulses when the current rises above saidminimum current point during periods of increasing and decreasingvoltages; and voltage level varying means responsive to said triggerpulses for changing the direction of voltage variation each time atrigger pulse is received.
 2. In the system of claim 1 wherein there isincluded means for establishing a predetermined initial voltage in acapacitor and means for producing a switching signal to initiate currentminimization, said combination including sample error pulse generatingmeans for entering a predetermined upper current limit signal into saidsample and hold means.
 3. The combination of claim 2 wherein triggerpulses are produced to reverse voltage direction when the motor currentreaches an initial upper current value.
 4. The method of varying voltagein a motor control system to minimize motor current comprising:decreasing voltage until motor current crosses either a minimum, ormaximum error level while current is increasing; increasing voltageuntil motor current crosses either a minimum, or maximum error levelwith current again increasing; and maintaining steady-state currentminimization by reversing the direction of voltage variation each timemotor current decreases to a minimum and then rises slightly above saidminimum by a maximum error value.
 5. A method for maintainingsteady-state current minimization in a motor control system comprising:establishing four control modes for the conditions of: (0) voltage andcurrent decreasing; (1) voltage decreasing with increasing current; (2)voltage increasing with decreasing current; and (3) voltage and currentboth increasing; operating a first circuit to hold a signal representingminimum current and a second circuit to follow current to the minimumduring mode (0); comparing a signal representing motor current with thesignal held by said second circuit during mode (1) and switching to mode(2) when motor current exceeds the minimum current held by said secondcircuit; operating said first circuit to follow motor current to saidminimum and said second circuit to hold the previously sampled minimumcurrent during mode (2) and comparing the signal of said first circuitwith said motor current representation to switch to mode (0) when motorcurrent exceeds the minimum current representation.
 6. In a systemwherein an output signal is caused to vary in response to a controlsignal such that the derivative of the output signal with respect to thecontrol signal changes sign as the control signal crosses a certainlevel, a method for automatically varying said control signal to causethe output signal to assume a minimum level where the derivative of saidoutput signal is substantially zero, said method comprising sampling thelevel of said output signal as its derivative decreases during periodsof increasing control signal and holding the zero derivative level ofsaid output signal during periods of decreasing control signal toproduce a first sample and hold sample; sampling the level of saidoutput signal as its derivative decreases during periods of decreasingcontrol signal and holding the zero derivative level of said outputsignal during period of increasing output signal to produce a secondsample and hold signal; and changing the direction of control signalvariation each time the output signal to level crosses the level ofeither of said sample and hold signals.
 7. The method of claim 6 whereinsaid output signal is motor current and said control signal is voltage.8. The method of claim 7 wherein first and second trigger pulses aregenerated when the current level sampled crosses the level of said firstand second sample and hold signals, respectively.